A known technique in a hybrid construction machine accurately brings an engine to a target operating state by causing a motor generator to assist the engine or to generate electricity through an as simple as possible configuration (see, for example, patent document 1). To achieve that task, the technique disclosed in patent document 1 incorporates a controller that obtains an engine speed corresponding to optimum torque of a set speed as a target speed and performs the following control so as to bring the engine close to an optimum operating state. Specifically, when the engine speed is lower than the target speed because of a large load torque on the engine, the controller causes the motor generator to operate as an electric motor according to a difference therebetween to thereby assist torque. When the engine speed is higher than the target speed because of a small load torque on the engine, the controller causes the motor generator to operate as a generator according to the difference therebetween to thereby store the generated electricity in a battery.
Another known control technique is, even with a sharp increase in a hydraulic load, to increase driving power supplied to a hydraulic pressure generator in response to the increase in the hydraulic load, while maintaining appropriate operating conditions of an internal combustion engine (see, for example, patent document 2). To achieve that task, the technique disclosed in patent document 2, while causing the internal combustion engine to drive the hydraulic pressure generator, sets a rate of increase in an output of the internal combustion engine to a predetermined value. An output upper limit value of the internal combustion engine obtained from the predetermined value of the rate of increase is then compared with a driving power requirement obtained from a hydraulic pressure output that the hydraulic pressure generator is required to produce. The output of the internal combustion engine is then controlled so as to be equal to, or smaller than, the output upper limit value when the driving power requirement exceeds the output upper limit value.